Experimental study of monazite/melt partitioning with implications for the REE, Th and U geochemistry of crustal rocks Aleksandr S. Stepanov ⁎, Joerg Hermann, Daniela Rubatto, Robert P. Rapp Research School of Earth Sciences, The Australian National University, Mills Road, Bld. 61, Canberra, 0200, ACT, Australia abstract article info Article history: Received 27 July 2011 Received in revised form 12 January 2012 Accepted 14 January 2012 Available online 24 January 2012 Editor: D.B. Dingwell Keywords: Monazite REE Trace element Fractionation We report the results of monazite/melt partitioning experiments conducted in the piston-cylinder apparatus at 10–50 kbar and 750–1200 °C, using a synthetic granite mix with approximately 10 wt.% H 2 O and doped with trace-elements in proportions corresponding to the composition of monazite. Monazite was produced in all experiments, generally in the form of small grains. Electron microprobe and laser ablation-ICP-MS an- alyses were carried out on the resulting “monazite–melt” mixes from these experiments, and the composi- tion of the crystallized monazite calculated using regression analysis. The concentrations of LREE and Th in the melts coexisting with monazite increase sharply with increasing temperature. Monazite solubility decreases by 35–40% as pressure increase from 10 to 30 kbar. Monazite sol- ubility in granitic melts with an Alumina Saturation Index above 0.85 and FeO +CaO +MgO b 3 wt.% can be described by the following equation: ln∑LREE ¼ 16:16 0:3 ð Þþ 0:23 0:07 ð Þ ffiffiffiffiffiffiffiffiffi H 2 O p -11494 410 ð Þ=T -19:4 4 ð ÞP=T þ lnX LREE mnz Where H 2 O is in weight percent, T is in Kelvin, P in kbar and ∑LREE is the sum of La–Sm in ppm; X mnz LREE is the molar ratio of LREE to the sum of all cations (REE, Th, U) in monazite. REE, Th, U, Y, V and As partition into monazite, whereas other trace elements (Li, Be, B, Sc, Ti, Mn, Sr, Zr, Nb, Ba, Hf, Ta and Pb) have monazite/melt partition coefficients less than unity. Monazite shows the greatest preference for LREE from La to Nd, with a progressive decrease in partition coefficients for Sm and the HREE. The partition coefficients for Th are 30% higher than those for the LREE, and Th/LREE ratios are inde- pendent of pressure and temperature. Partition coefficients for U are 4–23 times lower than for the LREE. The new experimental data provide a numerical basis for modeling the behavior of LREE, Th and U during fractional crystallization of granitic magmas, as well as the melting in the presence of monazite, both within the continental crust, and in subduction zones. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Monazite is a light rare earth elements (LREE) phosphate (LREE)PO 4 that typically occurs as an accessory mineral in metape- lites, granulites, peraluminous granites and is also common in car- bonatites and kimberlites (Lyakhovich and Barinskii, 1961; Overstreet, 1967; Bea, 1996; Jones et al., 1996; Spear and Pyle, 2010). Monazite is the major host for LREE, Th and U in low-Ca granites and metapelites (Overstreet, 1967; Bea, 1996). Monazite is also an important mineral for Th, U–Pb geochronology of crustal rocks. It has recently been proposed that monazite and/or allanite are important hosts for LREE, Th and U in deeply subducted crustal rocks (Plank, 2005; Klimm et al., 2008; Hermann and Rubatto, 2009; Plank et al., 2009; Skora and Blundy, 2010). Synthetic REE(PO 4 ) compounds crystallize in the monoclinic mon- azite structure for rare earths from La to Gd, in which REE ions are lo- cated in 9-coordinated polyhedra (Ni et al., 1995). Rare earths from Tb to Lu form crystals with a tetragonal structure that is isostructural with xenotime (YPO 4 ) and zircon (ZrSiO 4 ), in which ions reside in smal- ler 8-coordinated polyhedra (Ni et al., 1995). HREE and Y are the major impurities in monazite, representing the xenotime component. An im- miscibility gap exists between monazite and xenotime that shrinks with temperature (Gratz and Heinrich, 1997). Pressure and the pres- ence of Th increases the solubility of Y in the monazite structure (Gratz and Heinrich, 1997; Seydoux-Guillaume et al., 2002). Thorium usually comprises 1–10 wt.% of natural monazites, forming two differ- ent substitution mechanisms: huttonite (ThSiO 4 ) and cheralite (CaThPO 4 ). The endmember ThSiO 4 has two polymorphic modifications: thorite and huttonite. Tetragonal thorite is isostructural with zircon and Chemical Geology 300-301 (2012) 200–220 ⁎ Corresponding author. Tel.: + 61 2 612 55596; fax: +61 2 612 50941. E-mail address: stepanovas@gmail.com (A.S. Stepanov). 0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2012.01.007 Contents lists available at SciVerse ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo